One known heat-transfer-medium heating apparatus for heating a heat transfer medium in the related art is an apparatus using a PTC heater, in which a PTC thermistor element (PTC element) is used as a heat-generating element. A PTC heater exhibits a PTC thermistor characteristic in which the resistance increases as the temperature increases, and the temperature rise is moderated as the consumed current is controlled by the temperature rise, after which, the consumed current and the temperature of a heat-generating part reach a saturation region where they are stabilized. The PTC heater thus exhibits a self-controlled temperature property.
As described above, the PTC heater has a property in which the consumed current decreases as the temperature of the heater is increased, and when it reaches a constant temperature saturation region, the consumed current is stabilized at a low value. Utilizing this property, an advantage is afforded in that it is possible to reduce the electrical power consumption and to prevent an abnormal increase in the temperature of the heat-generating part.
Therefore, PTC heaters are used in a wide range of technical fields, and in the field of air conditioning, as disclosed in PTL 1, for example, a heat-transfer-medium heating apparatus in which a PTC heater is used as a heating apparatus for heating a heat transfer medium (in this case, engine coolant) supplied to a radiator, which heats air while an engine is stopped, has been proposed for a hybrid vehicle air-conditioning apparatus.
This heat-transfer-medium heating apparatus includes two heat-transfer-medium circulating boxes connected to each other in a fluid-tight manner, and a plate-shaped PTC heater is interposed between the two heat-transfer-medium circulating boxes so as to be in close contact therewith. A heat-transfer-medium circulating path is formed in each of the heat-transfer-medium circulating boxes so as to be positioned in the vicinity of the PTC heater. Thus, engine coolant flowing therethrough is heated with the PTC heater and circulated in a vehicle air conditioning apparatus to regulate the temperature in a vehicle cabin.
A board accommodating section, which is a separate compartment formed adjacent to the heat-transfer-medium circulating path, is provided in the heat-transfer-medium circulating box, and a control board that controls the PTC heater is installed in the board accommodating section. Heat-generating electrical components, such as IGBTs, FETs, and so forth, to be mounted on this control board are securely fastened with screws etc. to an electrical-component cooling wall member, which is disposed so as to separate the heat-transfer-medium circulating path and the board accommodating section, in a closely contacted manner such that the heat-generating electrical components are cooled by the engine coolant flowing through the heat-transfer-medium circulating path.